The first Impact I saw fly had the D.E.P.S. elevator linkage system. Worked great.

tommy s

All dressed up and nowhere to go!

A few shots of my IMPACT marooned in the workshop waiting for the snow to go....

Eric.

Matt,
I don't understand everything I know about servomechanisms but here are some basics:
To eliminate oscillations, the mechanical resonant frequency of the system should be ten times the servo loop bandwidth. That says we want the resonant frequency as high as possible. It is defined by

F= {the square root of [K(Jload+Jmotor)/Jload*Jmotor]}/2pi

F= resonant frequency
J= inertia
K=transmission stiffness

So we want K to be as stiff as possible and the ratio of load inertia to motor inertia to be low. The gear train makes the reflected load inertia look lower as would mechanical advantage in the linkage but we don't have much control over that in a pull pull system so we need to keep the inertia of the rudder low.

By the way we want the servo bandwidth high for good response. I suspect the band width is less than 3 HZ so we'd like the resonant frequency up above 30 HZ. Did anyone measure or estimate the frequency of the oscillations?

Our RC servo systems are non-linear from a servo analysis stand point so they are very difficult to analyze, but I bet we have some young engineers out there that could do it. I've got to go build my Impact.

Jim O

Thanks Jim, that makes sense. If we reduced the load's inertia (halving rudder weight), then the required res. frequency should increase. It assumes that the motor's inertia doesn't change. Or conversely, if the motor's inertia was increased (cored motor rather than coreless), the effect would be similar. In parallel, if K were increased by using a stiffer transmission set-up, resonant frequency should increase, hence the comments whether or not any one used a carbon push rod for rudder

I didn't measure the oscillation frequency on mine but if I could hazard a guess I'd say it was around 10 cycles per second (10 Hz)

MattK

Right. Not only should we keep the rudder light, but we should keep the cg of the rudder close to the hinge line. I was wondering if one could "honeycomb" the CA furnished rudder, like folks were doing to wing cores and cover the openings with monokote or silk and dope and have a low weight stiff rudder. Might have to put foam between the portions of the skins that remain. Is that clear and does it make sense? Maybe one of the folks that built a balsa rudder would like to experiment.

Jim O

The rudder on my Temptress is built up from sticks. Weight is less than half that of the Impact's (comparable size), and it's cg is nearer the hinge line. But I use the 8411 and it still oscillates on occasion. The oscillations only occur at neutral, never when te rudder is deflected. My pull cables are fairly loose so the transmission part of the equation may be affecting this outcome. I'll experiment with tigher cables but I believe the servo is the problem.

MattK

Ive had this oscillation w/ tight and loose cables. Worse on my Impact right now when the battery is charged. Also had it happen on a built up rudder plane and on ailerons w/ new servos.

If its persistant on rudder why not just dampen the system w/ some foam rubbing on the cables, worked for me on a previous plane.

I have a friend here in the uk using aWebra 160 2/stroke inhis Impact. does anyone out there have advice on location/size of cooling slots [a] in cowl, [b]in belly pan, [c] around pipe exit?

I woke up this AM with this rudder oscillation issue on my mind. Jim O and Troy Newman had given me several ideas to try. I put the 8411 back in and watched it “dance!�

But first I tried a field-fix that a JR service engineer had given me. I fitted an O-ring across the two pull-pull wires inside the fuselage. This was easy to do. Just undo the servo arm and slip the 3/4� O-ring over the wires. It stops the oscillation as soon as it starts, after you flick the rudder stick. Probably not a permanent fix but could get you out of trouble at a contest.

I replaced the ali-output shaft with a nylon shaft so that the gear train was all nylon. I still got the oscillation dance but not all of the time and it was tighter.

The next thing that I tried was tightening the wires. (Minus the O-ring) Not much change. Definitely looked like the spring tension in the wires was a contributing factor.

Next I took two wires of different thickness and strung them up with equal, but light loads. Then I added 10 lb to each and could see much more stretch in the thinner of the two wires. Definitely springy!

I had the thinner wire on the plane so off it came. The thicker wire did not stop the dance but it could easily be seen as significantly less of a swing, (2� down to 1/4�).

Then the multi-strand wire was traded out for a single piano wire on each side. After that there was no way that I could get the servo to dance. I tried more and less tensions. No change!

The weight difference was .09 oz

I put the O-ring back on to stop any pull-pull wire “humming� and that how it has been left.

[In case you think I have lost it, don’t forget that we can’t fly yet and the building is all done!]

Flight trials will tell the rest of the story.

Regards,

Eric.

Then the multi-strand wire was traded out for a single piano wire on each side. After that there was no way that I could get the servo to dance. I tried more and less tensions. No change!

The weight difference was .09 oz

I put the O-ring back on to stop any pull-pull wire “humming� and that how it has been left.

[In case you think I have lost it, don’t forget that we can’t fly yet and the building is all done!]

Flight trials will tell the rest of the story.

Regards,

Eric.

[/quote]

If I understand it you still have the solid piano wire on it and the o-ring. You not only got rid of the low frequency oscillation but the high frequency buzz.
The solid wires sounds easier and cheaper than the carbon rods.
Does the servo buzz with no load? The o-ring probably adds some friction which is okay as long as the servo centers okay.
Good work. Now we know there is a solution.
Jim O

For those "in the know" What is the difference between the original IMPACT, and the new color scheme kit.

I am building one of the ones red, yellow, white and blue with the red checkerboard on the bottom. It seems like there is more prefab stuff supplied. May not be true, just seems like it.

What is wrong with the rudder and robart hinge pins. This is the same exact settup I have used on my past 5 pattern planes without any problems. Looking at the rudder into the sun, you can see the 1/4" balsa extra inserts(inside the rudder) for the hinge points to glue to, and then you add these to the rudder post also. Not being critical of others just curious. Or, is this an improvement with the extra pieces to increase thickness for the hinge points to glue to?

The other information is very valuable and lots of it will be used.

Thanx

ed

Jim,
When I said "humming" I meant rattling of the wire or playing like a guitar string when the engine is running. The system is totally quiet at neutral. I might add the JR pull-pull servo brace if it still cold tomorrow!

I also forgot to state that I first reduced the mechanical advantage of the rudder by going a hole in on the servo arm.

The O-ring is not very tight on the wires and just acts like a damper.

Regards,

Captain Cabin fever...

Hi Eric,
After I sent that message I guessed that you were referring to the rods vibrating. Of course if one ran them through teflon tubing like the elevator rods that would take care of the problem.
Don't you get in trouble if the servo arm lengths aren't the same length as the rudder control horns in a pull -pull system?
I also noted that you moved the control horns up near the mid-point on the rudder which is a good idea. Did you have to add material inside the rudder and how did you do it?
I assume Troy suggested control line cable that isn't supposed to stretch. Did he have any other good suggestions?
Always trying to learn, Jim O

Jim,
I had already taken the LE off the rudder to fit a beveled edge for a more "conventional" hinge-line. This allowed me to plan and fit the rudder horn support. I also "keyed and notched" the horn plate a little. Then it was fitted with a chopped-fiber-expoxy cocktail and some gravity direction to the glue flow before it cured inside the rudder :-)

My geometry on the horn and servo arm did not have any length change propblems. You can check this very easily and accurately when the wire is piano-wire, by just connecting one side and giving a helping hand on the rudder during the push movement.

Troy suggested (advised) two ply U-control line wires due to their stiffness and non-stretchiness. He also said "go all nylon on the servo gear train".

What was also notable was how much less you could "flex" the rudder, by hand, when the system was turned on and at neutral. iIll reserve judgement on durability until I have had some stick time with this plane.

Regards,

Eric.

Jason,

Could you clarify on your pics showing the cooling layout in your Nats model? The first pic seems to show a horizontal false floor on the wing tube side of the vertical separator and the second pic doesn't. I can see where the airflow goes in the second pic - over the battery tray, down below the vertical plate, under the rudder servo plate and out the fancy shaped exit holes but I don't get the first pic.

Thanks

Malcolm

Impact Picture - Covered w/ vinyl

That looks awesome! What kind of vinyl did you use for that?

Vinyl the Sign shop had. It has about the same sheen as the finish on the plane. Designed the scheme in CAD and exported to a EP format that the signcutter used.

This was my first try, got some bubbles here and there. But overall looks better than any paint I would put on.

Brian, the trick to sticky vinyl application is liberal use of soapy water. Spray it onto the surface and then place the vinyl on it. I get the surface dripping wet with the solution. Soapy water lubricates the adhesive and allows some movement to its final spot. Then you can work the water and bubbles out with a cotton cloth, working from the middle, out.

MattK

Brian that's brilliant! I hope it holds up against the fuel. Matt is right about the soap. I use windex on the vinyl I put on my foamies. Once it drys it's down tight without bubbles.
Good luck with it, Mike

Looks good.
Steve

That looks fantastic!

Any idea how much weight the vinyl added? I was planning to paint mine next weekend, but if this is an option, I'll do that instead.

I weighed a stab half before and after, each stab half was about 0.2 ounce increase. I got lazy and didnt keep track of the rest.

The plane weight is 10#9oz w/ vinyl. I added a crutch as you see, using a 5 cell nimh 2250 battery, 9311sa aileron, 8411 rudder, 3421sa stabs, YS 1.6, ES pipe. NB soft mount.

Hey Brian,

Your Impact looks FANTASTIC! Best looking Impact I've seen.


Todd

Hi Guy's,

I've just repaired the fuselage on my Impact and have reached the point where I need to re-fit the rudder. Has anyone experimented with static balancing on the rudder? Ive checked mine and it will require 80g in the forward mass balancer to balance the rudder on the hinge line. it has been explained to me that balancing the rudder will eliminate the further possibility of rudder flutter. I'm reluctant to add that much weight to the fuselage rear end but equally don't relish the thought of building a balsa replacement, any ideas?

All replies appreciated!

Andy.